Abstinence, periodic: Also known as fertility awareness, natural family planning, and the rhythm method, this approach entails not having sexual intercourse on the days of a woman's menstrual cycle when she could become pregnant or using a barrier method (such as a condom, the diaphragm or a cervical cap) for birth control on those days.

Because a sperm may live in the female's reproductive tract for up to 7 days and the egg remains fertile for about 24 hours, a woman can get pregnant within a substantial window of time -- from 7 days before ovulation to 3 days after. Methods to approximate when a woman is fertile are usually based on the menstrual cycle, changes in cervical mucus, or changes in body temperature.